The present invention relates to the preparation of high thickness acrylic sheets utilizable in the manufacture of antinoise barriers.
In particular the present invention relates to transparent and high thickness polymethylmethacrylate sheets usually employed for antinoise barriers in motoways, viaducts, bridges, etc.
Antinoise barriers formed of high thickness polymethylmethacrylate sheets (10-25 mm) are known in the prior art. Also sheets with lower thicknesses (4-10 mm) are also known, which find minor application in the antinoise barriers field and are utilized especially in the luminous signs field, for preparing panels for applications in building both for indoors and outdoors and in the sanitary field (for instance shower box), etc. Such material results, indeed, particularly advantageous for the applications mentioned above due to the high resistance to external agents, the optimal optical properties and the high rigidity thereof. This last characteristic results even more particularly advantageous since the structures above mentioned must be subject to high pressures due, for example, to the high speed wind or to the close passage of cars and heavy transport.
It is known, however, that such polymethylmethacrylate sheets show a high speed of the flame propagation and a low fire-resistance. To improve the behaviour of acrylic manufactured articles in the presence of limited fires it is possible, therefore, to add fire-retardant compounds known in the art, such as halogenated compounds or phosphor-based compounds. However such compounds result harmful and require therefore special expedients in the production cycle with a consequent and substantial increase in costs.
Moreover the addition of the above mentioned compounds determines a sensitive worsening of the thermo-mechanical and optical properties of the PMMA sheets mentioned above. Besides these compounds are not, or are about to be no longer utilized because of the more and more restrictive laws of the various countries.
It is known in the art that to increase the flame resistance properties, a methylmethacrylate (PMMA) homopolymer obtained by casting processes, is utilized. With such process, indeed, sheets can be obtained with polymer having molecular weights higher than those obtained by extrusion. This is the reason why the above mentioned properties result superior.
It is known also that adding certain comonomers, such as for instance acrylates and styrene, increases the fire resistance of methylmethacrylate (MMA). See for example the patent EP-600.332. However alkyl-acrylates and styrene do not have very high resistances of flame propagation and moreover the obtained sheets show a too high deformation, even if smaller with respect to the homopolymer. This leads to a worsening of the optical properties of the sheet, which are, instead, properties to be maintained. All this is still more evident in the case of low thickness sheets.
It was therefore felt the need to have available sheets both for antinoise barriers and for the other applications indicated above allowing to obtain improved resistance properties to the flame propagation, a higher rigidity and a lower tendency to bending (distortion) or local deformation after exposition to the flame.
The Applicant has now surprisingly and unexpectedly found that by utilizing particular comonomers it is possible to prepare methylmethacrylate-based copolymers meeting the flame resistance properties, showing a lower tendency to warping while maintaining at the same time good thermo-mechanical properties and optimal optical properties.
An object of the present invention consists therefore in antinoise and flame-resistant sheets formed by supports and by sheets fixed thereon, cnharacterized in that the sheets, totally or at least on the surface, are formed by a methylmethacrylate copolymer and acrylic and/or methacrylic acid comprised between 1 and 20% by weight, preferably between 2 and 15% by weight, even more preferably between 3 and 10% by weight, when the copolymer is obtained by extrusion processes, or the content of said acids is comprised between 1 and 30% by weight, preferably between 2 and 25% by weight, even more preferably between 3 and 20% by weight, when the copolymer is obtained by casting. The antinoise barriers of the present invention are moreover characterized in that they do not contain antiflame compounds of halogenated type and/or phosphor-based compounds.
In the case of the copolymers of the invention obtained by casting, copolymers having weight average molecular weight (MW) ranging from about 20,000 to 2,000,000, preferably higher than 1,000,000, are obtained. If desired, also crosslinked copolymers can be obtained with the casting process to increase in particular the sheet chemical resistance.
When extrusion processes are utilized, the copolymer has lower molecular weight than the casting process, and i.e. generally from 20,000 to about 180,000, preferably between 80,000 and 130,000.
The supports to which the acrylic sheet is fixed are generally formed by metals, such as for intance aluminium.
The test that the antinoise and flame-resistant sheets must generally satisfy is described in ZTW-LSW 88 standard. It requires that a sheet, for instance 2 mxc3x971 mxc3x9715 mm, is inserted in a metal U-shaped structure bound on a cement support of 25 cm of height for the length of the whole structure. At the base of such structure two containers (30xc3x9720xc3x9730 cm), are placed at about ⅓ and ⅔ of the width of the structure, formed by wire netting wherein 600 g of wood wool are placed to which fire is given. After one hour the effects of the flame on the surface of the acrylic sheet are evaluated. The sheets of the invention satisfy the above mentioned test. The examples reported hereinafter to give a wider representation of the present invention, have been carried out on samples of more limited sizes.
The copolymer according to the present invention can be produced according to anyone of the polymerization processes known in the art. The polymerization in suspension or in bulk results to be particularly preferred. In the polymerization processes a chain transfer agent can be used, for instance mercaptanes-based, such as butyl-mercaptane (BM), 2-ethyl-hexyl-mercapto-propionate, octyl-mercaptane (OM), n-dodecyl-mercaptane, t-dodecyl-mercaptane or mixtures thereof, to obtain copolymers having lower molecular weight, particularly suitable for obtaining sheets by extrusion.
If desired during the preparation of the copolymer optional components can also be added, such as for instance UV adsorbers, as Tinuvin(copyright) P; lubricants, such as for instance stearic acid, palmitic acid and the corresponding alcohols; thermal stabilizers, such as for instance disulphides.
By the term acrylic sheets are meant sheets formed by homopolymers or copolymers of the acrylic and/or methacrylic acids and their esters wherein the alkylic group contains up to 8 carbon atoms, such as for instance methylmethacrylate, methylacrylate, ethylacrylate, butylmethacrylate, 2-t-hexyl-acrylate.
When the copolymer of the invention is utilized only on the surface of the sheets, obviously the flame-resistance properties are maintained, while the mechanical properties will depend on the composition of the intermediate sheet. These types of sheets are obtained, for instance, by co-extrusion.